Add new parameter --project_root to cpplint.py.

cpplint.py

Index: cpplint.py
diff --git a/cpplint.py b/cpplint.py
index ac19165b357d187d392ad7fb59f3f014c7ba3535..7ce70abeb6f756c731587bac55068a1355acc01d 100755
--- a/cpplint.py
+++ b/cpplint.py
@@ -524,6 +524,10 @@ _error_suppressions = {}
 # This is set by --root flag.
 _root = None
 
+# The project root directory. Used for deriving header guard CPP variable.
+# This is set by --project_root flag. Must be an absolute path.
+_project_root = None
+
 # The allowed line length of files.
 # This is set by --linelength flag.
 _line_length = 80
@@ -1065,6 +1069,10 @@ class FileInfo(object):
     if os.path.exists(fullname):
       project_dir = os.path.dirname(fullname)
 
+      if _project_root:
+        prefix = os.path.commonprefix([_project_root, project_dir])
+        return fullname[len(prefix) + 1:]
+
       if os.path.exists(os.path.join(project_dir, ".svn")):
         # If there's a .svn file in the current directory, we recursively look
         # up the directory tree for the top of the SVN checkout
@@ -6017,7 +6025,8 @@ def ParseArguments(args):
                                                  'filter=',
                                                  'root=',
                                                  'linelength=',
-                                                 'extensions='])
+                                                 'extensions=',
+                                                 'project_root='])
   except getopt.GetoptError:
     PrintUsage('Invalid arguments.')
 
@@ -6046,6 +6055,11 @@ def ParseArguments(args):
     elif opt == '--root':
       global _root
       _root = val
+    elif opt == '--project_root':
+      global _project_root
+      _project_root = val
+      if not os.path.isabs(_project_root):
+        PrintUsage('Project root must be an absolute path.')
     elif opt == '--linelength':
       global _line_length
       try: